This invention relates to titanium alloys and has particular reference to titanium alloys intended for use in conditions of high temperature and stress such as in aeronautical engines.
Alloys containing 6% aluminum, 5% zirconium, 0.5% molybdenum, 0.25% silicon, balance titanium, have been proposed for use in aircraft engines where service temperatures of up to 520.degree. C are encountered. Such alloys are described for example in British Patent No. 1,208,319. The alloys have to have great dimensional stability in such conditions as tolerances allowed in aircraft engines are very small. The alloys must possess very good high temperature strength and must be resistant to embrittlement on exposure to high temperatures. Embrittlement in this context means loss of ductility measured at room temperature before and after exposure to high temperatures. It is particularly important to note that the measurement after exposure should be taken with the surface of the alloy remaining. One test after exposure can be to measure the properties of the alloy with the oxidised surface removed. In practice, however, this is not sufficiently like real-life conditions since the alloy in use cannot have its surface removed during operation. It is important, therefore, that the alloy be resistant to oxidation and it has been discovered that a particular alloy combination is exceptionally resistant to oxidation as will be described below.
In addition to having oxidation resistance, the alloy must be ductile, it must have a high creep resistance, it must be forgeable, and it must be weldable as welding is frequently used in fabrication of parts made from such alloys. By "weldable" in the present context is meant that articles manufactured from the alloy can be used in the welded condition. It is not sufficient merely to be able to stick two pieces of metal together; the alloy in the post-welded condition after a suitable heat treatment must have properties virtually indistinguishable from the alloy in the pre-welded condition. The alloy must also be resistant to fatigue and must of course have a relatively high tensile strength.
Commercially useful alloys must also be resistant to ordering and the alloys must in use be stable at elevated temperatures. The alloys with which the present invention is concerned have a very fine alpha plate structure and precipitation is normally found at the alpha plate boundaries. The precipitation is thought to be influenced by the levels of molybdenum and niobium. The precipitation limits the application of components formed from the alloy both in terms of the temperature of use and the time at temperature. In order that the fatigue initiation characteristics are acceptable the aluminium equivalents in the alloy should be kept as low as possible since this will effect the dislocation behaviour in the alloy.
Improvements in any one or two properties of an alloy can usually be obtained by suitable modification to the composition or to the heat treatment. The difficulty, however, is to obtain these improvements and to maintain or even better the remaining properties of the alloy. For example, the tensile strength of an alloy can normally be improved by the addition of alloying elements, but this normally reduces the ductility of the alloy. The present invention is concerned with an alloy which has an acceptable balance of properties throughout the entire range.